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# Transformer riddle No.11 – Yyn0 Transformer

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• #189
Marcos

I was carrying out a fault current distribution for an earthing design for a primary or zone substation when I found the transformer to be used was Yyn0 (33/11 kV). Apparently the lines company here have many substations with these transformers.

I was carrying out a fault current distribution for an earthing design for a primary or zone substation when I found the transformer to be used was Yyn0 (33/11 kV). Apparently the lines company here have many substations with these transformers.

When I checked in the books the zero sequence in some books appear like a (i)"floating" impedance (with a value 3 to 10 times the positive impedance) while in other ones (ii) the equivalent circuit appears to be the same as a Dyn transformer.

Most of the books have the equivalent circuit as (i) in which case no zero sequence will flow for a phase to earth fault at the 11 kV feeders. Also when modelling this transformer with the software DigSILENT gives as result a very low value of current for a phase to earth fault.

The question is with this arrangement; Is there any current flowing for a phase to earth fault at the secondary? Having in mind that these substations are located mainly in semi-rural areas with overhead feeders is there any problems with this choice of transformer?

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• #1418
Hamid

a-Yes b-Generally no, with regard existing overhead line you can benefit from some advantages of this connection. 1- The zero-sequence impedance of a star winding will be very high if no delta winding is present. The actual value will depend on whether there is a low reluctance return path for the third-harmonic flux. For three-limb designs without a delta, where the return-flux path is through the air, the determining feature is usually the tank, and possibly the core support framework, where this flux creates a circulating current around the tank and/or core framework. The impedance of such winding arrangements is likely to be in the order of 75 to 200% of the positive-sequence impedance between primary and secondary windings. For five-limb cores and three-phase banks of single-phase units, the zero-sequence impedance will be the magnetizing impedance for the core configuration. Should a delta winding exist, then the third harmonic flux will create a circulating current around the delta, and the zero-sequence impedance is determined by the leakage field between the star and the delta windings. Five-limb transformers have their zero-sequence impedances substantially equal to their positive-sequence impedance between the relative star and delta windings. 2-Zero Sequence Circuit Modeling of Yyn0 and Dyn0 connection is very different as you see below: 3-According to above diagram, the Yyn0 connection is provide interconnection path for zero sequence current between two sides of transformer. Therefore each side zero sequence impedance (high impedance probably) can be transfer to opposite side via power transformer and that mentioned software can calculate them. 4- The most common characteristics of three-phase transformer connection grounded wye

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